You'd most likely to find a digital SPDIF optical out on a full-sized component-sized recorder. Very few portables have any digital outs at all whatsoever. If you have a portable with only analog outputs, you'll have to transfer the content the old-fashioned analog way - in real-time via the stereo analog connections to your computer system's sound card or onboard audio.
And hardly any external CD/DVD recorder these days have any analog or audio-only inputs whatsoever, so you'll have to transfer audio to the computer first before transferring it to CD. The portable no-computer-required burners can only burn to DVD (and do not support burns onto CD), so that would not have helped your dilemna.
And even if you could transfer your self-made MiniDisc recordings digitally through the SPDIF out, you'll still have to confront DRM issues. The DRM for MiniDisc allowed digital copies from the originally recorded disc to another digital medium, but would not even allow you to make any further digital copies from the copy that you've just made. (Simply put, the DRM for MiniDisc allows only one digital generation of copying from a self-recorded MD.) In some systems, DRM also applies to any transfers from the computer's hard drive to a disc, resulting in a write failure! As a result, the only way to completely bypass DRM if you have such a DRM-laden computer system would be to transfer the content to the computer using the old-fashioned analog connections.
And since you have a portable with only analog outputs, you'll have to find a stereo mini to stereo mini patch cable in order to use the player/recorder with a computer system since very few PC sound cards have any RCA inputs at all.